Phase locked loop tuning control system including a sync activated AFT signal seeking arrangement

ABSTRACT

In a phase locked loop PLL tuning control system for a voltage controlled local oscillator of the type wherein an automatic fine tuning voltage is selectively applied to the local oscillator after a PLL configuration has obtained lock at the nominal frequency value for the selected channel so as to accommodate frequency translated RF carriers, a circuit selectively activated by the average level of composite synchronization pulses indicative of improper tuning clamps the AFT voltage at a predetermined level. The predetermined level is selected to force the local oscillator frequency to be driven to an offset value at which an offset detector causes reinitiation of PLL control at discrete frequency step from the nominal value for the selected channel.

The present invention pertains to the field of phase locked loop (PLL)tuning control systems for a local oscillator including a mode switchingarrangement for selectively applying an automatic fine tuning (AFT)signal to the local oscillator after the PLL portion of the system islocked to accommodate frequency translations of received RF carriers.Such a tuning control system is described in U.S. Pat. No. 4,031,549,entitled "Television Tuning System with Provisions for Receiving RFCarriers at Non-Standard Frequencies", issued in the name of Rast et al.on June 21, 1977, and U.S. Pat. No. 4,109,283 entitled "FrequencyCounter for a Television Tuning System", issued in the name of Rast onAug. 22, 1978. These patents are incorporated by reference for theirdetailed disclosure or portions of an embodiment of the presentinvention to be described below.

In the tuning control system described in the above-identified patents,an offset detector determines when the frequency of the local oscillatorsignal, under AFT control, exceeds a predetermined offset, e.g., 1.25MHz, from its nominal value established under PLL control. When thelatter occurs, control of the local oscillator is returned to PLLcontrol and a programmable division factor of a counter within the PLLportion is changed to discretely shift the frequency of the localoscillator signal. This is done to enable the AFT signal to be used toacquire signals with frequency translations of approximately ±1.5 MHzusing an AFT discriminator with an unaided range of approximately ±0.5MHz.

In CATV (cable television) and MATV (master antenna) systems fordistributing television signals, television signals are often modulatedor remodulated onto RF carriers which have imprecise or non-standardfrequencies compared with respective broadcast RC carriers. Theimprecision may arise due to tolerance variations in the modulationapparatus of the distribution equipment or because of the intentionalselection of certain reference frequency signals for use in themodulation apparatus. Specifically, with respect to the lattersituation, since the frequency spacing between most television broadcastchannels allocated to the various television frequency bands is 6 MHz inthe United States, RF carriers for distribution in a CATV system areoften derived by generating harmonics of a 6 MHz reference frequencysignal. As a result, the cable RF carriers are translated in frequencyby -1.25 MHz from respective broadcast RF carriers. Specifically, forexample, the RF picture carrier for channel 2 occurs at 54 MHz (i.e.,the ninth harmonic of 6 MHz) rather than at 55.25 MHz, the standardbroadcast RF picture carrier frequency for channel 2.

While PLL tuning control systems of the type described above should becapable of handling such translations, difficulty has been found toarise because of interference or beat signal products which may beundesirably produced by non-linear signal processing circuits included,e.g., in the RF and IF sections of television receivers. This may beunderstood with reference to the amplitude versus frequency responsecharacteristic of a typical AFT discriminator employed in conventionaltelevision receivers graphically illustrated in FIG. 2. The responsecharacteristic includes a desired S-shaped control portion approximatelybetween ±0.5 MHz from 45.75 MHz. When an RF picture carrier istranslated in frequency by approximately -1.25 MHz, and the localoscillator frequency is at the broadcast nominal value, it has beenfound that a beat signal may be produced having a frequency between+1.25 MHz and +1.75 MHz from 45.75 MHz. It is believed that the beatsignal is due to the non-linear combination of the IF picture carrier ofthe selected channel and the IF sound carrier of the lower adjacentchannel. The beat signal causes the AFT response characteristic toexhibit another and undesired S-shaped control portion approximatelybetween +1.25 MHz and +1.75 MHz from 45.75 MHz.

Under the above conditions, after the PLL configuration has caused thefrequency of the local oscillator signal to be at its nominal value forthe selected channel and AFT control of the local oscillator has beeninitiated, a stable tuning condition in which the undesired beat signalis tuned can be established in response to the undesired control portionof the AFT signal between +1.25 MHz and +1.75 MHz from 45.75 MHz. Sinceduring the AFT control cycle the frequency of the local oscillatorsignal will not be offset by more than 1.25 MHz from its valuepreviously established under PLL control, PLL control is not reinitiatedwith the frequency of the local oscillator shifted as it should be tocorrect the improper tuning condition. The present invention is directedat correcting this type of problem.

According to the present invention, for use in a PLL tuning controlsystem of the type described above there is included a comparator fordetermining when a signal component derived from the IF signal, such asa picture synchronization component, has a predetermined conditioncorresponding to an improper tuning condition and an arrangementselectively activated by the comparator for causing the AFT signalcoupled to the local oscillator during an AFT control cycle to assume apredetermined level when the improper tuning condition occurs. Thepredetermined level is selected to drive the frequency of the localoscillator signal to exceed the predetermined frequency offset for theAFT control cycles. When the latter condition occurs, the offsetdetector causes the reinitiation of PLL control to establish thefrequency of the local oscillator signal at a discrete frequency stepaway from the frequency at which it was previously incorrectlyestablished under AFT control.

The present invention will be described in more detail with reference tothe accompanying Drawing in which:

FIG. 1 shows in block diagram form an embodiment of the presentinvention incorporated in a color television receiver;

FIG. 2 graphically illustrates the AFT discriminator amplitude versusfrequency response characteristic referred to above; and

FIG. 3 shows in schematic form a circuit implementation of a portion ofthe embodiment of the present invention shown in FIG. 1.

The color television receiver shown in FIG. 1 includes three selectivelyenabled tuner sections 1, 2 and 3 for heterodyning broadcast and cableRF carriers with local oscillator signals having appropriate frequenciesfor the selected channel to produce respective IF signals. The IF signalgenerated by the enabled one of tuners 1, 2 and 3 is filtered andamplified by an IF section 4. The IF signal processed by IF section 4includes picture, color and sound carriers. In the United States, the IFpicture, color and sound carriers have respective nominal frequencies of45.75 MHz, 41.17 MHz and 41.25 MHz. In the United States the localoscillator (LO) frequencies, f_(LO), are higher than the RF carrierfrequencies, f_(RF). The following equation defines the relationshipbetween the RF, IF and LO frequencies.

    f.sub.IF =f.sub.LO -f.sub.RF                               (1)

It will be noted that the IF carrier frequencies vary inversely withrespect to the RF carrier frequencies.

Luminance and synchronization signals are derived from the picturecarrier by luminance and synchronization sections 5 and 6, respectively.Color signals are derived from the color carrier by chrominance section7. Sound signals are derived from the sound carrier by sound section 8.Luminance section 5 controls the detail, contrast and brightness of apicture produced by a picture tube 9. Chrominance section 7 controls thecolor properties, i.e., saturation and hue, of the picture. Afterprocessing, the luminance and chrominance signals (the lattercorresponding to the primary colors red, green and blue) are combined bya picture tube driver 10 and applied to picture tube 9. Synchronizationsection 6 derives composite horizontal and vertical synchronizationpulses. Vertical and horizontal deflection sections 11 and 12 respond torespective synchronization pulses to generate electron beam deflectionsignals applied to deflection cores 13 associated with picture tube 9.Sound section 8 provides audio signals to a speaker 14.

In the United States, RF carriers heterodyned by tuners 1, 2 and 3reside in the frequency bands indicated in the following table.

    ______________________________________                                                       RF                  LO                                                        RANGE    CHANNEL    RANGE                                      BAND           (MHz)    NUMBER     (MHz)                                      ______________________________________                                        low VHF broadcast (LV)                                                                        54-88   2-6        101-129                                    midband cable (M)                                                                             90-174  (A-5)-I    137-215                                    high VHF broadcast (HV)                                                                      174-216   7-13      221-257                                    super band cable (S)                                                                         216-402  J-(W + 17) 263-443                                    UHF broadcast (U)                                                                            470-890  14-83      517-931                                    ______________________________________                                    

Tuner 1 heterodynes RF carriers in the VHF broadcast and midband cablebands. Tuner 2 heterodynes RF carriers in the super band cable bands.Tuner 3 heterodynes RF carriers in the UHF broadcast band. Tuners 1, 2and 3 are selectively enabled to operate in response to respective bandswitching signals V/M, S and U generated by a phase locked loop (PLL)tuning control system to be described below in accordance with the bandof the selected channel.

A VHF antenna network (not shown) or a cable installation (not shown) isselectively connected by a user to an input unit 15 which includes ahigh pass filter for passing RF carriers in the VHF broadcast andsuperband and midband cable bands and rejecting signals outside thesebands. Input unit 15 is connected to tuner 1 and tuner 2. A UHF antennanetwork (not shown) is connected to an input unit 16 which includes animpedance transforming network for transforming balanced impedanceconfiguration of the UHF antenna network to an unbalanced configurationsubstantially matching the input of tuner 3.

Each of tuners 1, 2 and 3 includes an RF section "a" for selecting aparticular RF carrier associated with a selected channel, a localoscillator (LO) "b" for generating a local oscillator signal having theappropriate frequency for heterodyning the selected RF carrier and amixer "c" for combining the selected RF carrier and local oscillatorsignal to produce the respective IF signal. In each of tuner 1, 2 and 3,RF section "a" and local oscillator "b" include tuned circuits havingtheir frequency response controlled in response to the magnitude of atuning voltage generated by the PLL tuning control system in accordancewith the channel number and band of the selected channel. Typically, thetuned circuits include the parallel combination of an inductive circuitand a varactor diode which is reverse biased to exhibit a capacitivereactance having a value determined by the magnitude of the tuningvoltage. Since VHF broadcast and midband cable tuner 1 must tune over afrequency range too large to permit the use of a single inductor in eachof its tuned circuits, its tuned circuits include respective inductorselection arrangements (not shown) for selecting the appropriateinductor configuration in response to respective band switching signalsLV, M and HV.

Channels are selected by a channel selection unit 17, e.g., including acalculator-like keyboard and/or up and down channel scanning keys, forgenerating first and second groups of BCD (binary coded decimal) signalsrepresenting the tens and units decimal digits, respectively, of thechannel number corresponding to a selected channel.

The channel numbers for the various bands are indicated in the abovetable. It will be noted that the cable channels are identified byletters as well as numbers. An air/cable switch 18 is provided to enablethe channel selection unit 17 to be used to select cable channels aswell as "off-the-air" broadcast channels. When switch 18 is in an "air"position, operation of channel selection unit 17 causes the generationof the first and second groups of BCD signals corresponding to the tensand units digit of the channel number of the respective broadcastchannel and the subsequent tuning of the respective broadcast RFcarrier. When switch 18 is in a "cable" position, operation of channelselection unit 17 normally causing the generation of the first andsecond groups of BCD signals corresponding to the tens and units digitof the channel number of a broadcast channel and the subsequent tuningof the respective broadcast RF carrier, causes the tuning of arespective cable-carrier which has been assigned, as indicated on achart supplied with the receiver, to the selected broadcast channelnumber.

The BCD signals generated by channel selection unit 17 are applied to achannel number display unit 19, a band selector 20 and a divider (÷N)control unit 21 of the PLL tuning control system. Band selector 20,which is also responsive to the position of air/cable switch 18,generates band selection signals representative of the band of theselected channel. In addition to band selection signals for the LV, M,HV, S and U bands, band selection unit 15 generates a V/M band selectionsignal when the selected channel is in one of the LV, HV or M bands anda U/S when the selected channel is in one of the U or S bands. As notedabove, the V/M, U and S band selection signal activates the tuners 1, 2and 3, respectively, to heterodyne the corresponding RF carriers and theLV, M and HV band selection signals select the respective inductorconfigurations of the tuned circuits of tuner 1.

As described above, the frequencies cable RF carriers may be translatedwith respect to respective broadcast RF carriers. The PLL tuning controlsystem is of the type described above for accommodating nonstandardfrequency as well as standard frequency RF carriers. The PLL tuningsystem includes: a PLL configuration for causing tuners 1, 2 and 3 to betuned to standard frequencies corresponding to the standard frequency RFbroadcast carriers associated with selected channels; an AFTconfiguration for causing tuners 1, 2 and 3 to be tuned so as to reducedeviations between the frequency of the picture carrier of the IF signaland its nominal value, e.g., 45.75 MHz, which may occur due to thereception of nonstandard frequency RF carriers; and a mode selectionarrangement for selectively enabling the operations of either the PLL orAFT configurations.

The PLL configuration includes a prescaler 22 for dividing the frequencyof the local oscillator signal selectively applied to it by the enabledone of tuners 1, 2, 3 by a division factor K sufficient to bring thefrequency of the resultant signal within the operating range of afollowing programmable divider 23. Since the frequency range of thelocal oscillator signals of tuner 1 is considerably lower than thefrequency range of local oscillator signals of tuners 2 and 3, (e.g., inthe United States, by a factor of 4), division factor K is controlled inresponse to the U/S and V/M bandswitching signals. Programmable divider23 divides the frequency of the output signal of prescaler 22 by aprogrammable factor N determined by divider (÷N) control unit 21 inaccordance with the channel number and band of the selected channel. Aphase comparator 24 compares the phase and/or frequency of the outputsignal of programmable divider 18 and a reference frequency signal togenerate an "error" signal having pulses with widths and transitiondirections related to the magnitude and sense, respectively, of thephase and/or frequency deviations between the output signal ofprogrammable divider 23 and the reference frequency signal. Thereference frequency signal is derived by a frequency divider (÷R) 25which divides the frequency of the output signal of a crystal oscillator26. The division factor R of programmable divider 25 may also becontrolled in accordance with the frequency band of the selected channelin response to the U/S and V/M band selection signals.

The error pulses generated by pulse comparator 24 are selectivelyapplied to the input of a low pass filter (LPF) 27 comprising anintegrator by a mode switch 28 of the mode selection arrangement to befurther described below. In response to the error pulses a tuningvoltage is generated at the output of LPF 27 and applied to the enabledone of tuners 1, 2 and 3. During the subsequent operation of the PLLconfiguration, the magnitude of the tuning control voltage changes inaccordance with the phase and frequency deviations between thefrequency-divided local oscillator signal and the reference frequencysignal in a sense to reduce the deviations. When the deviations havebeen minimized, the frequency of the local oscillator signal f_(LO) willhave a value proportional to the frequency of the crystal oscillatorsignal, f_(XTAL), by the following expression:

    f.sub.LO =N K/R f.sub.XTAL                                 (2)

Desirably, the factor K/R f_(XTAL) is made equal to 1 MHz, so that theprogrammable division factor N of divider 23 is equal, in MHz, to thefrequency of the local oscillator signal.

The AFT configuration includes an AFT discriminator 29 of theconventional type for generating an AFT signal having a generallyS-shaped amplitude versus frequency characteristic representing themagnitude and sense of deviation of the frequency of the IF picturecarrier from its nominal value, e.g., 45.75 MHz, as shown in FIG. 2. TheAFT signal is selectively applied to the input LPF 27 by an AFT switch30, the function of which will be described below, and mode switch 28.In response, the tuning control voltage developed at the output of LPF27 is responsive to the AFT signal. During the subsequent operation ofthe AFT configuration, the frequency of the local oscillator signal isadjusted from its value established during the previous operating cycleof the PLL configuration to correct for any deviations of the frequencyof the IF picture carrier from 45.75 MHz due to correspondingtranslations of the frequency of the received RF carrier.

The mode switching apparatus for selectively coupling either the outputof phase comparator 24 or AFT discriminator 29 to LPF 27 includes modeswitch 28 comprising a double thrown, single pole electronic switchingdevice and a mode control unit 31 for controlling the "position" of modeswitch 28. Mode control unit 31 includes: a flip flop (not shown) forgenerating a "mode" signal having a first binary level for causing modeswitch 28 to apply the output signal of phase comparator 24 to LPF 27and a second binary level for causing mode switch 28 to apply the outputsignal of AFT discriminator 29 to LPF 27; and combinational logic (notshown) for controlling the state of the flip flop in response to signalsgenerated by channel selection unit 17, a lock detector 32, and anoffset detector 33.

When a new channel is selected, a "change" pulse is generated by channelselection unit 17. In response to the "change" pulse, the "mode" signalis set to its first level. In response to the first level, mode switch28 applies the error signal generated by phase comparator 24 to LPF 27and thereby enables the operation of the PLL configuration.

When the phase and/or frequency deviations between the frequency dividedlocal oscillator signal and reference frequency signal have beensubstantially minimized, the pulses of the error signal generated byphase comparator 24 will have relatively short durations. Lock detector32 examines the durations of the error pulses and when the lattercondition occurs generates a "lock" signal. Mode control unit 31responds to the "lock" signal and causes the "mode" signal to be set toits second level. In response to one second level, mode switch 28applies the AFT signal generated by AFT discriminator 29 to LPF 27 andthereby enables the operation of the AFT configuration. It is assumed atthis point that the state of AFT switch 29 is such that the AFT signalis applied to mode switch 28.

At any time after the initiation of the AFT arrangement, should thefrequency of the local oscillator signal applied from the enabled one oftuners 1, 2 and 3 be offset from the value established during theoperation of the PLL configuration by a predetermined amount, e.g., 1.25MHz, offset detector 33 detects the occurrence and generates an "offset"signal. Offset detector 33 is enabled to operate in response to thesecond level of the "mode" signal. As described in detail in theaforementioned Rast patent, offset detector 33 includes a counter (notshown) and a switch (not shown) controlled in response to a "gating"signal generated by reference divider (÷R) 25 to selectively applyoutput signals of prescaler 22 to the counter. The count accumulated bythe counter during a predetermined time interval of the "gating" signalis indicative of the frequency offset of the local oscillator signalunder AFT control. In response to the "offset" signal, the "mode" signalis reset to its first level thereby ending the operation of the AFTconfiguration and reestablishing the operation of the PLL configuration.Local oscillator frequency offsets greater than 1.25 MHz are to beavoided to avoid tuning the receiver to the sound carrier of the loweradjacent channel, with reference to the IF range, which sound carrier isseparated from the picture carrier of the selected channel by 1.5 MHz.

Certain cable and master antenna installations may provide nonstandardfrequency RF carriers translated in frequency from respective standardfrequency RF carriers typically provided by broadcast transmitters bymore than the AFT control range, e.g., approximately ±0.5 MHz.Accordingly, to expand to tuning range of the PLL tuning control system,a "step" counter 34 is included for incrementally changing the value ofN in response to the generation of the "offset" signal when theoperation of the PLL configuration is reinitiated.

Specifically, in response to the first generation of the "offset" signalduring the first AFT operating cycle after a new channel has beenselected, step counter 34 is set to a predetermined state which causesthe value of N to be increased by 1 with respect to the nominal valuefor the selected channel and the operation of the PLL configuration isreinitiated. Accordingly, the frequency of the local oscillator signalis increased by 1 MHz with respect to the nominal frequency for theselected channel. Thereafter, when the "lock" signal is again generated,the operation of the AFT arrangement is initiated for the second time.In this manner, the tuning control system is capable of locating andtuning nonstandard frequency RF carriers which may be translated infrequency from respective standard frequency RF carriers by +1 ±0.5 MHz(±0.5 MHz being the control range of the AFT signal).

If no RF carrier is located for the increased value of N, a second"offset" signal will be generated during the second AFT operating cyclewhen the frequency of the local oscillator signal is caused to be morethan 1.25 MHz from the value established previously under the control ofthe PLL configuration. In response to the second generation of the"offset" signal, step counter 34 is set to a state causing the value ofN to be decreased by 1 with respect to its nominal value for theselected channel and the operation of the PLL configuration is againreinitiated. Accordingly, the frequency of the local oscillator signalis decreased by 1 MHz with respect to the nominal frequency for theselected channel. Thereafter, when the "lock" signal is again generated,the operation of the AFT arrangement is initiated for the third time. Inthis manner, the tuning control system is capable of locating and tuningnonstandard frequency RF carriers which may be translated in frequencyfrom respective standard frequency RF carriers by -1 ±0.5 MHz.

If it is desired to expand the search range of the PLL tuning controlsystem, this may be accomplished by changing the step sizes of N or byadding additional steps. Thus, a fourth PLL control operation may beinitiated if an RF carrier is not located by the end of the third AFTcontrol operation with the value of N changed by another step, e.g., -2.

As previously described with reference to FIG. 2, it has been found thatwhen the RF picture carrier is translated approximately -1.25 MHz fromits standard value, an undesired beat signal may be producedapproximately between +1.25 and +1.75 MHz from 45.75 MHz. Depending onthe amplitude of the beat signal, this may cause AFT discriminator 29 toexhibit an undesired control range between +1.25 and +1.75 from 45.75MHz. Accordingly, under these conditions, when the AFT control cycle isinitiated after the first PLL control cycle, a tuning condition may beestablished in which the undesired beat signal is erroneously tuned inresponse to the portion of the AFT signal between +1.25 and +1.75 MHzfrom 45.75 MHz. Absent AFT switch 30 and composite synchronizationcomparator 35, to be described below, this erroneous tuning conditionwould be incorrectly maintained. This is so because although the IFpicture carrier may be more than +1.25 MHz from 45.75 MHz, the frequencyof the local oscillator signal will not have been offset by more than1.25 MHz from its starting value, i.e., the value established during thefirst PLL control cycle. Under the latter conditions, absent AFT switch30 and composite synchronization comparator 35, offset detector 33 willnot generate an "offset" signal and the tuning control systemerroneously remains under AFT control with the undesired beat signaltuned.

AFT switch 30 and composite synchronization comparator 35 cooperate tocorrect improper tuning conditions of the type describe above.Specifically, composite synchronization comparator 35 detects impropertuning conditions of the type described above by examining the compositesynchronization pulses generated by synchronization section 6. When anRF carrier is properly tuned, composite synchronization pulses have apredetermined duty cycle. It has been found that when an improper signalis tuned, there are many more erroneous pulses compared with the normalnumber of composite synchronization pulses per unit of time generated bysynchronization section 6 due to the presence of IF noise components.Composite synchronization comparator 35 detects this occurrence and inresponse generates an "improper tuning" signal. AFT switch 30 isactivated in response to the "improper tuning" signal and causes thepredetermined offset of 1.25 MHz to be exceeded. Specifically, AFTswitch 30 responds to the "improper tuning" signal and decouples the AFTsignal generated by AFT discriminator 29 from mode switch 28 and insteadcouples a predetermined voltage PV to mode switch 28. The polarity andmagnitude of predetermined voltage PV are selected so that the tuningvoltage driven to a corresponding polarity and magnitude condition atwhich the frequency of the local oscillator signal is assured to havemore than the predetermined offset from its nominal value establishedduring the first PLL control cycle.

In the improper tuning condition described above, during the first AFTcontrol cycle, the improper tuning condition will be detected bycomposite synchronization comparator 35 and in response AFT switch 30will be activated to cause the predetermined offset of +1.25 to beexceeded during the first AFT control cycle. This in turn causes thesecond PLL control cycle to be initiated in which the frequency of localoscillator signal is shifted by +1 MHz from its nominal valueestablished during the first PLL control cycle. The frequency shift of+1 MHz of the local oscillator signal in combination with the frequencytranslation of the RF carrier of -1.25 MHz causes a correspondingfrequency shift of the IF signal of +2.25 from 45.75 MHz (see equation(1)). At the latter shifted frequency, an improper tuning condition willagain be detected by horizontal synchronization detector 35 and inresponse AFT switch 30 will be activated to cause the predeterminedoffset of 1.25 MHz to be exceeded during the second AFT control cycle.In turn the third PLL control cycle is initiated.

During the third PLL control cycle the frequency of the local oscillatorsignal is shifted by -1 MHz from its nominal value established duringthe first PLL control cycle. The frequency shift of -1 MHz of the localoscillator in combination with the frequency translation of the RFcarrier of -1.25 MHz causes a corresponding frequency shift of the IFsignal of +0.25 from 45.75 MHz. Since the IF picture carrier will now bewithin the desired AFT control range between ±0.5 MHz from 45.75, it canbe tuned during the third AFT control cycle.

It has been found, for the most part, that the composite synchronizationpulses have approximately the correct duty cycle as long as the IFpicture carrier is within the proper control range of the AFT signal,i.e., approximately between ±0.5 MHz from 45.75 MHz. Accordingly,composite synchronization comparator 35 has been found not to generatean "improper tuning" signal and thereby not improperly terminate an AFTcontrol cycle, for the most part, unless an improper tuning condition ofthe type described above occurs. Thus, RF carriers which are translatedin frequency so that the corresponding IF signal is within the AFTcontrol range of any of the IF frequency steps corresponding to thesteps of N may be located and tuned as described above.

Since the improper tuning condition described above due to frequencytranslations of RF carriers are most likely to occur only when the RFcarriers are provided by a cable distribution system, compositesynchronization comparator 35 is disabled by air/cable switch 18 whenthe latter is in the "air" position. In addition, since "off-the-air" RFcarriers may be "weaker", i.e., have lower amplitudes and contain morenoise components than cable RF carriers, it is desirable to disablecomposite synchronization comparator 35 when "off-the-air" RF carriersare being received to prevent false triggering in response to noisecomponents.

The circuit implementation of AFT switch 30 and compositesynchronization comparator 35 shown in FIG. 3 is arranged to cooperatewith LPF 27, as shown, comprising an inverting amplifier configuration.The circuit implementations of AFT switch 30 and compositesynchronization comparator 35 commonly include a bipolar NPN transistor36 functioning both as a threshold comparator and switch element. Thebase of transistor 36 is connected through a current limiting resistor37 to the junction of a resistor 38 and a capacitor 39 comprising anaverage detector for developing a voltage across capacitor 39representing the average value of the composite synchronization pulses.

The collector of transistor 36 is connected through a diode 40 and aresistor 41 to AFT discriminator 29. Resistors 41, 42 and 43 form avoltage divider network for scaling the AFT voltage applied to thecollector of transistor 36 and mode switch 31. The AFT voltage generatedby AFT discriminator 29 varies relative to a positive voltage levelhaving a magnitude sufficiently large so that transistor 36 is incondition to be rendered conductive when its base to emitter voltage isgreater than approximately +0.7 volts. Diode 40 is poled to preventtransistor 36 from being rendered conductive in response tonegative-directed portions of the AFT voltage.

The emitter of transistor 36 is connected through a resistor 44 toair/cable switch 18. The emitter of transistor 36 is also connectedthrough a resistor 45 to a source of positive supply voltage SV. Whenair/cable switch 18 is in the "air" position, substantially all ofsupply voltage SV is applied to the emitter of transistor 36 throughresistor 45. The value of SV is selected to prevent transistor 36 frombeing rendered conductive. When air/cable switch 18 is in the "cable"position, the voltage at the junction of resistors 44 and 45 is reducedfrom SV to a voltage TV. Voltage TV determines the threshold voltage atwhich transistor 36 is rendered conductive. When the average value ofthe horizontal synchronization pulses exceed TV by more thanapproximately 0.7 volt, transistor 36 is rendered conductive. Diode 40is also rendered conductive at this time. As a result, the junction ofresistors 41 and 42 is clamped to a relatively fixed positive voltagesubstantially determined by resistors 41, 42, 43, 44 and 45 and thevoltage drops between the anode and cathode of diode 40 and between thecollector and emitter of transistor 36. The latter voltage replaces thenormally varying AFT voltage. Resistors 41, 42, 43, 44 and 45 areselected so that voltage PV developed at the junction of resistors 42and 43 and applied to the mode switch 31 when transistor 36 isconductive causes the local oscillator signal to have a frequency offsetgreater than 1.25 MHz from its nominal value established during theprevious PLL control cycle as described above. A relatively large valueresistor 46 couples air/cable switch 18 to band selector 20.

The following is a list of component and voltage values for theimplementations which have been found experimentally satisfactory whenemployed in an RCA television receiver type CTC-108 which includes a PLLtuning control system of general type described above and which isdescribed in "RCA Television Service Data--CTC 108, File 1980 C-5",published by RCA Corporation, Indianapolis, Ind., hereby incorporated byreference.

    ______________________________________                                        parameter         value                                                       ______________________________________                                        resistor 37       100 kilohms                                                 resistor 38       51 kilohms                                                  capacitor 39      0.47 microfarads                                            resistor 41       12 kilohms                                                  resistor 42       15 kilohms                                                  resistor 43       20 kilohms                                                  voltage SV        5.1 volts DC                                                resistor 44       1 kilohm                                                    resistor 45       1.5 kilohm                                                  resistor 46       10 kilohms                                                  AFT voltage       see typical amplitudes                                                        in FIG. 2                                                   ______________________________________                                    

The present arrangement for clamping the AFT voltage to a predeterminedlevel to cause reinitiation of PLL control when improper tuning occurshas special advantage when the tuning system is substantiallyincorported in an integrated circuit or in a number of integratedcircuits as is the case in the above identified RCA CTC-108 chassis.That is, since the present arrangement operates on the AFT signal priorto use within an integrated circuit, as is indicated by dotted line 47in FIG. 3, it may readily be added to an existing integrated circuitconfiguration comprising a tuning control system of the general typedescribed above.

While the present invention has been described with reference to aparticular embodiment and by way of example of its operation withreference to a particular improper tuning condition, it is not intendedthat its construction be limited to the described embodiment or its usebe limited to the correction to the described improper tuning conditionbut rather be defined by the following claims.

What is claimed is:
 1. In a multiband television receiver includingchannel selection means for selecting channels; RF means for selectingan RF carrier corresponding to a selected channel in response to themagnitude of a tuning control signal; local oscillator means forgenerating a local oscillator signal with a frequency corresponding tothe selected channel; mixer means for combining said selected RF carrierand said local oscillator signal to produce an IF signal including apicture carrier having a frequency with a predetermined nominal value;an automatic fine tuning (AFT) discriminator for generating an AFTsignal having an amplitude representing the deviation of the frequencyof siad picture carrier from said nominal value; and a synchronizationprocessing section for generating at least one synchronization signal;apparatus comprising:reference frequency means for generating areference frequency signal; comparison means for generating an errorsignal representing the phase and/or frequency deviation between saidlocal oscillator signal and a comparison signal having a frequencysubstantially equal to the product of a programmable factor and thefrequency of said reference signal; programmable factor control meansfor controlling the programmable factor, said programmable factorcontrol means setting said programmable factor in response to thechannel number of the selected channel when a new channel is selected;lock detector means for determining when said error signal has apredetermined condition; tuning signal generating means for generatingsaid tuning signal in response to one of said error and AFT signals;mode selection means for selectively coupling one of said AFT signal andsaid error signal to said tuning signal generating means, said modeselection means being responsive to said channel selection means forcoupling said error signal to said tuning control means when saidchannel is changed and being responsive to said lock detector means tocouple said AFT signal to said tuning signal generating means when saiderror signal has said predetermined condition; offset detector means fordetermining when said local oscillator signal has a predeterminedfrequency offset from the frequency established when said error signalwas coupled to said tuning signal generating means after said AFT signalis coupled to said tuning signal generating means; said mode selectionmeans being responsive to said offset detector means for coupling saiderror signal to said tuning signal generating means when said localoscillator signal has said predetermined frequency offset; saidprogrammable factor control means being responsive to said offsetdetector means for changing said programmable factor by a predeterminedincrement when said local oscillator signal has said predeterminedfrequency offset; synchronization detector means for determining whensaid synchronization signal has a predetermined condition; and AFTcontrol means responsive to said sync detector means for decoupling saidAFT signal from said tuning signal generating means and instead couplinga predetermined level selected to cause said local oscillator signal tohave said predetermined value to said tuning signal generating meanswhen said synchronization signal has said predetermined condition. 2.The apparatus recited in claim 1 wherein:said AFT control means includesa conductor coupled between said AFT discriminator and said modeselection means for normally coupling said AFT signal to said modeselection means; and clamp means responsive to said synchronizationdetector means for selectively establishing said predetermined level atsaid conductor when said synchronization signal has said predeterminedcondition.
 3. The apparatus recited in claim 2 wherein:said AFT controlmeans further includes switch means for selectively disabling said clampmeans from being responsive to said synchronization detector means. 4.The apparatus recited in claim 3 wherein:said switch means is manuallyoperable to allow selection of the frequency band of said selected RFcarrier to be one of a broadcast or cable band; and said clamp means isdisabled from being responsive to said synchronization detector meanswhen said switch means is set to cause the frequency band of saidselected RF carrier to be in said broadcast band.
 5. The apparatusrecited in claim 2 wherein:said synchronization section generateshorizontal and vertical synchronization pulses as a compositesynchronization signal; and said synchronization detector means isresponsive to said composite synchronization pulses.
 6. The apparatusrecited in claim 5 wherein:said synchronization detector includesaverage detector means for detecting the average value of saidhorizontal synchronization pulses; and threshold detector means coupledto said average detector means for activating said clamp means when theaverage value of said horizontal synchronization pulses has apredetermined value.
 7. The apparatus recited in claim 6 wherein:saidclamp means includes voltage developing means for developing a voltageat said predetermined level; a bipolar transistor having a base, acollector and an emitter, said base being connected to saidsynchronization detector means so that said transistor is renderedconductive when the average value of said horizontal synchronizationpulses has said predetermined value, one of said collector and emitterbeing connected to said voltage developing means, the other one of saidcollector and emitter being connected to said conductor.
 8. Theapparatus recited in claim 7 wherein:said threshold means includes thesemiconductor junction between the base and emitter of said transistor.9. The apparatus recited in claim 8 wherein:said AFT control meansfurther includes switch means connected to said one of said base,collector and emitter for selectively disabling said transistor frombeing rendered conductive in response to the average value of saidhorizontal synchronization pulses.
 10. The apparatus recited in claim 9wherein:said switch means is manually operable to allow selection of thefrequency band of said selected RF carrier to be one of a broadcast orcable band; and said transistor is disabled from being renderedconductive in response to the average value of said horizontalsynchronization pulses when said switch means is set to cause thefrequency band of said selected RF carrier to be in said broadcast band.